High-voltage apparatus



R. H. VARIAN HIGH VOLTAGE APPARATUS Filed Dec. 7, 1942 2 Sheets-Sheet lFlG. l.

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I I I ll 1 26a Y 2 1 I60. l4 PHASE SHIFTING Z NETWORK g l Z PULSER 3 .r84 5 PULSER LOW POWER U.H.F. 36

SOURCE m '3 x O 2 TIME WE V FIG. 3.

P P P PULSER lo INVENTOR Russsu. H. VARIAN ATTORNEY Jan. 8, 1946. R. H.VARIAN 2,392,380

HIGH VOLTAGE APPARATUS Filed Dec. '7, 1942 2 Sheets-Sheet 2 I ll V 1PULSER FIG. 5.

-IL3O TRANSFORMER ".9 I I r 1/ INVENTOR RUSSELL H. VARIAN 30 ATTORNEYPatented Jan. 8, 1946 HIGH-VOLTAGE APPARATUS Russell H. Varian, Wantagh,N. Y., assignor to Sperry Gyroscope Company, Inc., Brooklyn,

N. Y., a corporation of New York Application December 7, 1942, SerialNo. 468,306

32 Claims.

This invention relates to the production of high velocity electronstreams and particularly to ultra high frequency apparatus and methodsfor producing high voltage electron streams.

As the preferred embodiment of the invention, apparatus for producinghigh voltage electron streams which collide with a target and producepenetrating X-rays will be described, but it will be understood thatthis disclosure is by way of example only and does not limit the scopeof the invention which embraces the production of high velocityelectrons for any purpose. In my preferred apparatus I obtain highvoltage X-rays by passing an electron beam through a suitable highvoltage ultra high frequency field produced by a space resonator as ofthe type shown in U. S. Letters Patent No. 2,259,690 in which I amco-inventor, and then causing the beam to strike against a metal targetto produce X-rays.

Earlier patents relating to ultra high frequency apparatus employingspace resonant devices of the general type to be considered relative tomy specific disclosure herein, as, for example, Hansen No. 2,190,712 andWebster et al. No. 2,227,372, have suggested the possibilities ofproducing X- rays therefrom, and this invention may be essentiallyregarded as a development resulting from those suggestions.

In view of the above, it is a major object of the invention to providenovel methods and apparatus employing ultra high frequency oscillationsfor accelerating electrons to high velocities. A specific noveladaptation of this object is the production of high voltages forproducing X- rays and the like.

A further object of the invention is to provide novel methods andapparatus for producing high speed electrons wherein pulses of electronsto be accelerated are passed through a recurrent oscillating field onlywhen those oscillations have attained'a maximum level.

A further object of the invention is to provide novel methods andapparatus for producing high speed electrons wherein pulses of electronsare passed through an oscillating electromagnetic field in such phasewith the oscillations of the field that the electrons are given veryhigh and preferably maximum acceleration during passage through thefield.

Another object of the invention is to provide novel methods andapparatus for producing high speed electrons wherein pulses of electronsare injected into a recurrent oscillating electromagnetic fieldsubstantially only when the oscillations have reached maximum level, andwherein the electrons are introduced into the field in predeterminedphase to be accelerated by the high frequency oscillations of the field.Preferably the oscillating field is contained within a hollow conductiveresonator.

A further object of the invention is to provide a. novel method andapparatus for producing high speed electrons wherein the electrons in apulsating electron beam are velocity grouped prior to introduction intoan accelerating high frequency oscillating electromagnetic field so asto be in prescribed phase with said field for maximum acceleration.

It is a further object of the invention to provide a novel method andapparatus for producing high speed electrons wherein a pulsatingelectron beam is swung through a selected path for introducing electronsinto a high intensity oscillating electromagnetic field only inpredetermined phase to be accelerated by the oscillations of the field.

It is a further object of the invention to provide novel methods andapparatus for producing high speed electrons wherein direct currentpulses are supplied to periodically excite an oscillatingelectromagnetic field, and electron pulses for injection into the fieldare controlled proportionally to the rate of change of said excitingcurrent pulses so as to be introduced into said field in predeterminedphase with the oscillation periods. Preferably this control isaccomplished by a novel transformer arrangement in series with theexciting current pulse supp y.

A further object of the invention is to provide a novel apparatus forproducing high speed electrons wherein an electron beam adapted to beinjected into a high intensity oscillating electromagnetic acceleratingfield is preliminarily subjected to the action of a related highintensity oscillating electromagnetic field for introducing theelectrons into said accelerating field in proper phase for maximumacceleration. Preferably this object is attained at least in part byaltering the potential of the apparatus producing said preliminaryaction as a unit with respect to the accelerating apparatus, so as toobtain a uniform electron beam.

A further object of the invention is to provide a mobile X-ray or likegenerator having a novel hollow resonator of relatively small physicaldimensions and having relatively low shunt impedance to its excitingvoltage.

A further object of the invention is to provide a novel hollow resonatorstructure having a single resonant circuit and shaped for maximumeniciency of oscillation.

A further object of the invention is to provide novel excitationarrangements for a hollow resonator adapted to be periodically excitedto a relatively high oscillation voltage level by pulsating energy,wherein the resonator is excited to a predetermined intermediate levelby synchronized pulses of energy introduced into the resonatorsufliciently prior to the pulses producing high voltage oscillation. tobuild up the field to that intermediate level by the time said pulsesproducing high voltage oscillation are introduced, and thereby keeppower consumption to a minimum. Preferably the pulses producingexcitation to the intermediate level are of somewhat longer durationthan those producing the high voltage oscillation level.

It is a further object of the invention to provide novel ultra highfrequency apparatus embodying an oscillator which is excited to at leasta substantial minimum level of excitation and then periodically raisedto a relatively high level.

of oscillation, wherein the power required to attain a very highaccelerating voltage applied to the the minimum level and impulsivelyproduce the higher level is a minimum.

A further object of the invention is to provide a novel ultra highfrequency hollow resonator apparatus wherein a substantially annularbeam is injected into the resonator to set up a high intensityoscillating field and a second electron beam, preferably central withthe annular beam, is injected into the resonator field.

Further objects of the invention will presently appear as thedescription proceeds in connection with the appended claims and theannexed drawings, wherein Fig. 1 is a partially diagrammatic viewincluding an axial section of a preferred apparatus embodying theprinciples of the invention and having a hollow resonator foraccelerating the electrons to X-ray voltages.

Fig. 1A is a graphic representation of the timing of the auxiliaryexcitation and main power pulses preferred in the invention.

Figs. 2 and 3 are graphical representations illustrating operation ofthe apparatus of Fig. 1.

Fig. 4 is a diagrammatic representation of the manner in which theapparatus of Fig. 1 may be used.

Fig. 5 is a partially diagrammatic view of a further embodiment of theinvention wherein loosely coupled transformer means is provided forsynchronizing the pulses of electrons with the oscillation periods ofthe resonator.

Fig. 6 illustrates in like manner a further embodiment of the inventionwherein a swinging electron beam is injected into the resonator only inproper phase relation to the high frequency oscillations in theresonator.

Fig. 7 illustrates a further form of resonator which may be used in anyembodiment of the invention.

Research on space resonator devices leading to the above-identifiedpatents established that the shunt impedance (Rs) of a space resonator,which is comparable to the resistance of a parallel LC circuit atresonance in ordinary radio computations, varies in magnitude directlyas the square root of the resonant wavelength, or inversely as thesquare root of the oscillation frequency. This means that theoscillation voltages obtainable from a continuous driving current ofspecified magnitude supplied from the cathode through the resonator arealso proportional to the square root of the resonant wavelength, sincethis oscillation voltage is proportional to the product of the drivingcurrent and the resonator shunt impedance. Hence, since the shuntimpedance increases as the resonator size increases, it has heretoforebeen assumed that to obtain high oscillation voltages most economicallyrequired the use of space resonators of relatively large dimensionshaving high shunt impedances and relatively low oscillating frequencies.

My present invention includes the d o y electrons prior to striking thetarget to generate the X-rays. To maintain such a high voltagecontinuously in an oscillation generator including a hollow resonatorwould result in excessive power dissipation. It has been foundadvantageous to successively excite the hollow resonator at the requiredhigh voltage for very short time intervals separated by relatively longtime intervals during which the voltage within'the resonator is verymuch smaller. In this manner. the required high oscillation voltagwithin the hollow resonator is obtained without the high powerconsumption which would be necessary to maintain the high voltagecontinuously.

Thus, according to the invention, I propose to produce intermittent buthighly periodic X-ray emission by applying power to high frequencyoscillator generating apparatus in direct current pulses of suitableduration. For minimum power consumption it is desirable that each powerpulse'be of minimum duration for building up the oscillations to fulloperating voltage, but the pulses must not be shorter than the build-uptime of the resonator. This difliculty can be overcome by employing anindependent auxiliary oscillator to continuously introduce a smallamount of energy at resonant frequency into the resonator so that theresonator is caused to maintain oscillation at a relatively low voltagelevel between pulses instead of reducing to zero. Under such conditionsit is possible to use driving pulses of considerably shorter durationand therefore of appreciably less power than would be essential forreaching full oscillation on each driving pulse in the absence of theauxiliary oscillator. The power saving will exceed the power suppliedfrom the auxiliary oscillator. In my invention I therefore supply powerpulses at suitable intervals for speedily fully oscillating a partiallyexcited oscillator as will be later explained in detail.

In general, with reference to the invention, substantially any powerpulse duration including very short pulses, may be obtained byappropriate design in the pulser. Although such short power pulses makeit possible to achieve the required high voltage in the resonatorwithout high average power input, the instantaneous high power valuesduring each pulse are, of course, not reduced. This would ordinarilyrequire very large feeder lines, which is undesirable. However, in theinvention I employ relatively small feeder lines continuously supplyingpower to a pulser circuit adapted to store the incoming power in aconveniently designed condenser, and then discharge the accumulatedpower in very short Deriodic bursts through the resonator.

Such condensers are apt to be the most expensive and heaviest part ofthe equipment, both of which considerations are essential in design ofcommercial apparatus. Design usually begins with the assumption that acertain amount of money only is available for the condenser and the restof the apparatus is correlated for most efficient operation with thatcondenser. My investigations have shown that best results are obtained,and the highest voltages useful for accelerating electrons to formX-rays may be provided from a condenser of given size, throughassociation of the condenser with a very high frequency oscillatorhaving a small closed hollow resonator, as in the invention hereindescribed. This can be shown by the following considerations.

Where a hollow resonator is powered by an electron current from a Pulsecircuit having a storage condenser, the amount of charge required fromthe condenser during each pulse and hence the condenser size) isdetermined by the product of the current through the resonator and thetime the current must be maintained to build up the amplitude of theoscillations in the resonator from the maintained low level to fullvalue. This factor of course determines the size of the condenser aswell as establishing a practical minimum pulse duration, since the peakcurrent is limited by the nature of the cathode used.

For a particular resonator shunt impedance and cathode emission current,however, just above the value required to start oscillations in theresonator, and especially during the initial major portion of thebuildup period, the amplitude of oscillation and hence the oscillationvoltage in the resonator during each pulse will increase in the order ofone power of e in seconds, or will be multiplied by e every seconds,where Q is 21r times the ratio of the energy stored to energy loss percycle in the resonator, f is the frequency of oscillation at resonanceand e is the base of the natural logarithms. Actually the excitingcurrent is usually somewhat above the starting value, and so the rate ofbuildup is somewhat faster. Q varies substantially inverselyproportionally to the square root of the frequency I, because of theincreased losses at higher frequencies due to decreased skin depth. Thebuildup time T from low level to a predetermined high level oscillationis proportional to and, therefore, varies inversely as the 3/2 power ofI. For a given high oscillation voltage, the beam current required isinversely proportional to the shunt impedance, and is thus proportionalto the square root of f. The condenser size depends on the product ofthe current and the buildup time, and therefore is inverselyproportional to the frequency f, for a given oscillation voltage. Withhigher frequencies, and a given condenser size, the oscillation voltageobtained durin a given pulse duration will therefore increase, so thatunder these conditions higher oscillation voltages are obtainable, as isdesired. This is a distinctive feature of the invention, and is contraryto prior art teachings which used larger resonators for obtaining highervoltages, and hence used lower frequencies for such higher voltages.

However, there is an upper limit on the frequencies which may be used indevices of the present type. Thus, the amount of cathod current requiredduring each pulse to attain a given steady oscillation voltage isproportional to the square root of the frequency of oscillation. Thecathode area available for passage of this current through a resonatorof particular shape is inversely proportional to the square of theoscillation frequency since the linear dimensions of the resonator varyin inverse proportion to frequency. There is therefore a maximumfrequency determined by the obtainable cathode current density. Thisrestriction on the available maximum oscillation frequency is also apractical limitation on the oscillation voltage and condenser size.

The duration of the shortest pulse it is practical to generate alsoforms a limitation on the maximum frequency. Since the oscillationvoltage must build up to maximum value during the pulse, a minimum valueof Q for this shortest pulse is determined. This value of Q depends onfrequency, and fixes the upper practical frequency limit.

We now consider the necessary relations between the low level ormoderate oscillations and the high level or maximum oscillation voltage.If the low level oscillations are too low, excessive high level power isrequired from the pulsed beam to build up oscillations to the maximumlevel. If the low level oscillations are too high, more power isexpended in these low level oscillations than is saved by pulsing thehigh power excitation. The optimum condition is found when the powerinput sustaining the low level oscillations is equal to the pulsed highlevel power. If, for example. each pulse lasts Vouc of the total timebetween pulses, then FL) V0 should be of the order of /1000, where V1 isthe moderate oscillation-sustaining voltage between pulses, and V0 isthe maximum oscillation voltage during each pulse. If

is very much less than /1000, more power from the low level source willbe dissipated in the resonator between pulses than is consumed duringthe high level pulse, whereas if I seconds, the build-up time isapproximatel I which is about as short an oscillation build-up time ashas been found to be practical under actual conditions. Thus, theduration of each pulse is preferably of the order of seconds.

Still greater economy in power may be had if the sustaining power isalso pulsed, but with a longer duration and lower amplitude than thedriving pulse to achieve the high voltage, aswill be explained indetail,

Since a large amount of power must be transferred into the resonator ina very short time and it is desirable to keep the cathode drivingvoltage as reasonably low as possible, a very large exciting currentmustbe put into the resonator. This requires that the grid area for thedrivin beam must be relatively large and this larg grid area makes theshunt impedance that the resonator has toward the driving beamrelatively low. As will appear, I preferably employ a space resonator ofsuch dimensions as to obtain maximum grid area for a particular resonantfrequency.

The fact that the resonator used in the apparatus of the invention haslow shunt impedance toward the driving beam is further advantageous inthat it increases the ratio of available driving power to energy loss.Thus, assuming the resonator Q to remain unchanged, the energydissipated or lost because of the existence of the capacity of the gridsis proportional to AVGZ, where A is the projected grid area and Vo isthe voltage between the grids. Although this might indicate that higheroscillation voltages would be obtained by resonators having high shuntimpedance toward the driving voltage, as a practical matter emissioncurrent density from the cathode has a fixed maximum after which thepower deliverable to the resonator is proportional to AVG.

The efficiency or ratio of driving power to loss is, therefore,inversely proportional to Va. The smaller Va is, the greater efliciencyis therefore obtained. For a given driving power, this requires A to beincreased as Vc is decreased, so

' that a low shunt impedance resonator is required.

In actual practice, Q does not remain absolutely constant and tends todecrease with lower shunt impedance, thus diminishing the aboveadvantage slightly, but this is outweighed by the other considerationsexplained. The low shunt impedance resonator is thus furtheradvantageous because it permits a relatively high current withoutrequiring high current density. The high input current which thus may beused reduces the required input voltage for a desired power input,reduces slightly the time required for building up the oscillationvoltage during each pulse, and operates adequately with a low shuntimpedance resonator.

As aresult of the above analysis, it is clear that according to theinvention an efficient high frequency X-ray or similar apparatusrequiring high speed electrons may be designed employing a cavityresonator of small physical dimensions having low shunt impedance towardthe driving beam and alarge projected grid area. This resonator operatesvery efliciently with high current input which means that the cathodedriving voltage is maintained at a minimum for given power input. I havediscovered thata condenser capable of delivering high current atrelatively low voltage may be employed in the pulse circuit fordelivering the required power pulses for intermittent excitation of theelectric field for producing sufficient acceleration of an electron beamto produce X-rays.

I have further discovered that the high voltages incident to X-rayoperation may be handled by an ultra, high frequency resonator device oflow shunt impedance for the driving beam and high impedance for theelectrons that produce X-rays, and that because of the inherent limitingfactors above discussed it is neither essential-nor efficient to uselarge resonators to cluding a working head In of convenient size.

A hollow closed generally cylindrical metal resonator body ll, formedwith concaved reentrant central end portions l2, has anupstanding'substantially cylindrical extension wa1l l3 at one end onwhich is sealed 2. glass cover I through a gas-tight sealing rimdesignated at [5.

Cover l4, extension wall l3'and the side and bottom walls of resonator Nform a gas-tight enclosure which is evacuated to a'high vacuum! Anannular cathode I6 is suitably supported within the enclosure inalignment with an an-" nular perforated grid l1 formed or seated in theupper end of resonator ll. Within resonator II a partition l8 parallelto the resonator ends supports a concentric annular tubular form l9provided at opposite ends with annular perforated grids 2| and 22-axially aligned with grid l1. Be-' low grid 22 is a reticulated annularclosed grid 23 carried by the bottom wall of resonator II, This grid 23is used to suppress secondary electron emission from the bottom wall.

This annular cathode and grid construction may be of the form disclosedin Fig. 5 of my Patent No. 2,242,275, issued May 20, 1941, or in Fig. 12of U. S. Letters Patent No. 2,259,690, issued October 21, 1941, to whichreference is hereby made for details of structure and operation.

Centrally of upper dished wall portion I2 is seated a perforated grid 24for admitting the electron beam from cathode 25 into the interior ofresonator ll. Cathode 25 is provided with a conical focusing electrode26 which focuses electrons emitted from cathode 25 through apreliminaryperforated focusing grid 21 and into the resonator.

Heater filaments 28 and 29 for the resonator and X-ray cathodes l6 and25, respectively, are energized by suitable transformer coupling to asupply line at 3| as indicated. The leads supplying the cathodes aresealed in suitable gas tight joints where they pass through the wall ofcover M,

In the exposed end wall of resonator I I, I preferably provide awater-cooled target 32 of suitable design for emission of X-raysproduced by impact of the high voltage beam. A suitable annular coolingattachment for the resonator is indicated at 30.

The power circuit for the resonator includes a pulse generator ofsuitable design 33 embodying a condenser 34 for periodically storingcontinuous current energy and dischargin at intervals for intermittentlyproducing high voltage pulses of required length and power. As shown at[6a in Fig. 1, the pulse circuit is connected directly to cathode iii. Atransformer 35 in series with the condenser 34 is connected at 25a tocathode 25 so that electron pulses for producing X- rays are injectedinto the resonator at the timethat the oscillating electromagnetic fieldwithin the resonator has built to full oscillation voltage. A rectifier30 is connected in shunt with the transformer 35 for a purpose to bedescribed.

An auxiliary independent source of ultra high frequency energy 36 iscoupled as by a concentric line 31 to resonator l I, within the upperchamber aseaeeo which it terminates in the usual energy couping loop 38.This is to provide moderate exc tation of the field between grids l1 and2| so that the oscillations in resonator II will not have to build upfrom zero voltage during each pulse from pulser 33. The frequency ofthis auxiliary source 36 is equal to the resonant frequency of resonatorII. The oscillation voltage in resonator ll maintained by oscillator 36may be about $6 of the peak voltage. The voltage driving cathode l6 maybe in the neighborhood of 50,000 volts.

If desired, oscillator 36 may deliver energy continuously to resonator Hso that it tends to continuously sustain the moderate voltage level atall times, both during and between the periods that main pulses arereceived from cathode I6.

I have. however, discovered that a considerable further saving in powermay be effected if the power supplied to source 36 is also delivered inpulses related to the driving pulses supplied to cathode l6 as showndiagrammatically in Fig, 1A. This may be accomplished by providing foroscillator 36 any suitable energy source 84, such as a knownmultivibratcr assembly, having a. pulsating square topped wave output,such as at 80 in Fig. 1A. The output of pulser 33 to cathode I6 isconnected by lead 8| to pulser source 64 through a suitable phaseshifting or dela network 32 adapted to control the output of pulser 84so that each pulse therefrom is related in time to the main power pulse83 from cathode IS in the manner disclosed in Fig. 1A.

Oscillator 36 is thereby periodically excited by pulser 84, andtransmission line 31 feeds corresponding pulses of ultra high frequencyenergy to provide excitation of resonator II to moderate level by thetime that the main pulse is injected by cathode I 6. The invention is ofsuflicient scope to cover any suitable arrangement for obtaining thistiming of the periodic energy pulses.

The dotted line in Fig. 1A represents an optimum condition for relationof the pulser which may be obtained by apparatus of suitable design, butthe square topped wave arrangement shown in the solid lines represents agood and readily obtained practical embodiment of this phase of theinvention.

Where the moderate exciting voltage is pulsed. it may be considerablyhigher than 6 of the peak voltage if desired.

As indicated by Fig. 1A, pulse 80 is preferably of considerably longerduration than main pulse 83 and is not continuous throughout the entiretime between pulses 83. The reason that pulse 80 is of longer durationthan pulse 83 is that ordinarily the time required to build up theoscillation voltage in the resonator from zero to the moderateexcitation level is longer than the time required to build it up furtherto peak voltage, because the voltage rise from zero to moderateexcitation requires passage through more powers of e and thus occurs ata slower rate, this rate depending on the increase in excitation to beobtained.

It is essential only that the field produced by pulse 80 be built up tothe required moderate excitation level by the time that the mainexcitation pulse from cathode I6 is started. This requirement means thatpulse 80 must be initiated and maintained only during the periodrequired to provide that moderate excitation level in resonator I I atthe proper time, Consequently there is a period between pulses when noenergy, either from pulser 83 or source 36, is supplied to resonator II.

In substance therefore I have provided for periodically excitingresonator II to moderate excitation level so timed that wnn the mainpulse is injected from cathode IS, the field within resonator ll need bebuilt up only from that moderate level to peak value. Any suitableapparatus for so timing the periodic moderate excitation of theresonator with the main pulses maybe used without departing from thespirit of the invention.

Resonator II is preferably of annular form as illustrated in order toprovide the large grid areas essential to handling large currents duringhigh power operation at minimum driving voltages. This shape gives muchincreased grid area with attendant low shunt impedance. In a practicalembodiment, I use a ten or twelve square centimeter annular cathode areain a resonator having a resonant wavelength of about ten centimeters.The driving power pulse duration is preferably about f seconds as aboveexplained. Preferably, I use an oxide-coated cathode which has anemission current density of several amperes per square centimeter. Thus,each pulse could be made to have a peak current of 30-40 amperes whichwould represent a required power input of 1500 to 2000 kilowatts with50,000 volts applied to the cathode. However, since the pulses last only5 of the total time between pulses, the required average power input isonly about 1.5 or 2 kilowatts.

In normal operation of the apparatus shown in Fig. l, resonator llbecomes a self-excited oscillator when the cathode I6 is properlyenergized. An electron stream passes from energized annular cathode l6through the space between grids H and 2|. Normally the electric fieldbetween grids I1 and 2| would become oscillatory to at least some degreedue to thermal agitation of the electrons or other non-uniform electronvelocity or density conditions, even in the absence of oscillations fromsource 36, and this oscillatory field would produce b anching ofelectrons in the stream passing through the drift space within form 19and arriving between grids 22 and 23. A resonance would then immediatelyautomatically build up in resonator ll if the flight time through thedrift space is correct and become effective to increase the smalloscillatory field between grids I1 and 2 1, thus further increasingelectron bunching in the stream. This cyclic process would continueuntil the oscillations in resonator ll would have built up to a steadyhigh value. For further details as to this manner of self-excitedoscillation, reference is made to my said Patent No. 2,242,275.

In the apparatus of the invention I am not forced to rely on such minuteoscillations as thermal agitation of the electrons for initialoscillations, as oscillator 36 supplies suficient energy to start theoscillations at a relatively higher moderate level as above described.Once the oscillations start building up upon application of the pulsefrom cathode l6, however, the cyclic buildup process proceeds asdescribed during the pulse until full oscillation amplitude or level isreached.

In Fig. 2, the solid line illustrates the nature of the voltage pulsesdelivered from pulser 33, which is also the nature of the pulsesdelivered by cathode l6. Fig. 3 illustrates the nature of the voltageoutput from the secondary winding of transformer 35. There are twoinduced voltage pulses in the transformer secondary during each rimarypulse, one when the primary pulse is building up, and one when itsubsides, but they are oppositely directed.

The pulse P that makes cathode 25 more positive with respect to groundedgrid 21 will of course cause no electron emission therefrom, but thesecond pulse N which is in the reverse direction making cathode 25 morenegative causes the required high increase in electron emissiontherefrom. As shown by the dotted lines in Fig, 2, the impedance of thetransformer may cause a lag in the pulse shut-off. This is relativelyunimportant, but it is generally important to prevent this impedancefrom causing a similar delayed rise when the pulse is initiated. This isaccomplished by shunting out the transformer primary at the start of thepulse N, as by rectifier 30', but not at the close of the pulse, as thenthe current would be in the blocking direction of the rectifier.

Each relatively short delayed electron pulse from cathode 25,corresponding to a pulse N in Fig. 3, is timed to be injected into theresonator whenthe electromagnetic held within the resonator has beenbuilt up by the associated exciting power pulse from cathode l to fullvoltage and maximum oscillation.

Thus, with each pulse from condenser 34, a large amount of power istransferred from cathode |6 into resonator II which automatically buildsup the electromagnetic field therein to oscillate at ultra highfrequency. Provision of auxiliary oscillator 36 insures that resonator Iis very quickly built up to full oscillation at resonant frequencyduring each pulse.

The high voltage available in the oscillating electric field withinresonator II is employed to accelerate the electrons emitted by cathode25 to velocities suitable for producing very penetrating X-rays. Theflight time of the electrons between wall portions I: of the resonatoris preferably equal to less than one-half cycle. In this manner, forpower inputs in the range of 1.5 to 2 kilowatts as above described, Imay obtain electron velocities of hundreds of thousands of electronvolts. Because of the penetrating character of the X-rays generated whenthe high voltage electron beam strikes target 32, the X-rays are capableof penetrating the target and appear outside head III as indicated.

Working head |0 of the X-ray generator, because of the low impedancerequirements for the space resonator, may comprise an enclosure not morethan six inches across its largest dimension. In actual use it can beconnected by a suitable flexible shielded cable 39 to the relativelybulky pulser 33 which may be mounted on a convenient work bench, asshown in Fig. 4. Since all of the high voltage and other lines leadingto head "I are within cable 39, the head may be safely and convenientlymoved about at will for exploring a casting 40 or like industrial use,and because of its small compact nature may be introduced into cavitiesnot accessible to previously known bulky X-ray generators. The wireswithin cable 39 need be insulated for only about 50,000 volts and shouldbe so arranged that the cable has but small reactance, to avoid unduecomplications arising from transmission of the sharp pulses carriedthereby. Although the excitation in the generator is some hundreds ofthousands of volts, no insulation therefor is needed as that highvoltage exists only inside the resonator.

Fig. 5 illustrates another embodiment of the invention having specialcathode arrangements and circuits for insuring that the electrons to beaccelerated are injected into the resonator not only in pulses timed inproper phase with each pulse from cathode |6 as shown in Figs. 2 and 3,but also in electron groups arranged in proper phase with the highfrequency oscillations, for accelerating the electrons to maximum speed.

Resonator II is preferably an annular figure of revolution similar toresonator II in Fig. 1, with an annular grid I! being provided in itsinner end wall in alignment with annular cathode I6. Wall 40 is providedon its axis with a medial depressed portion 4| having a central entrancegrid 42 for resonator Grid 42 is in axial alignment with cathode 25which supplies the X-ray producing electrons. A small cylindrical metalresonator shell 43 is coaxial with resonator I A hollow tube 44 projectsfrom the interior of shell 43 in close proximity to wall 40. An entrancegrid 45 is provided in resonator 43 to receive the electrons projectedby cathode 25, and grids 46 and 41 are provided at the ends of tube 44.A focusing grid 48 is mounted/on wall 40 in the path of the electronstream between grids. Cathode 25 and grids 45, 46, 41, 48, and 42 areall aligned with resonators II and 43.

Pulser 33' is connected to the primary 49 of a transformer having fivesecondary windings 5|, 52, 53, 54 and 55. Windings 5|, 54 and 55 areloosely coupled to primary 49, while windings 52 and 53 are closelycoupled to primary 49 for reasons which will appear later. Cathodefilaments 28 and 29 are connected across a battery 56 through windings52-55, and a lead 51 connects resonator 43 through winding 5| to thepositive terminal of a battery 59 in series with battery 56.

The interior of resonator 43 is electrically coupled with the interiorof resonator II' by a high frequency transmission line such as the usualconcentric line and loop connections indicated at 59, 6|, so that highfrequency energy from resonator is transmitted to the interior ofresonator 43. An isolating insulating member 60 which functionssimilarly to a blocking condenser insulates line 59 from line 6| so thatthey may be at different unidirectional potentials, but passes the highfrequency energy from resonator to resonator 43. It is not shown indetail as it is not a part of this invention.

In operation of the apparatus of Fig. 5, pulser 33 delivers high powerhigh voltage direct current pulses as in Fig. 1, but here the pulses areimpressed across primary 49.

Cathode l6 is excited by the closely coupled secondaries 52 and 53 andreceives a broad pulse such as shown at in Fig. 2. Cathode 25 is excitedby the loosely coupled secondaries 54 and 55 and delivers electrons togrid 45 only near the close of each broad pulse in brief pulses timedcorresponding to N in Fig. 3.

The simultaneous identical pulses initiated in each of the looselycoupled secondary windings 5|, 54 and 55 insure that structures 25, 43,44 and 41 all change potential simultaneously by the same amount, thusinsuring that a constant stream of electrons exits through grid 4'! toenter resonator during each pulse corresponding to N in Fig. 3. Sincethere is no voltage between grids 41 and 48 when there is no pulse frompulser 33, no current will enter resonator I except on the pulse N shownin Fig. 3. Resonator 43 together with drift tube 44 causes velocitygrouping of the electrons passing therethrough during pulse N in theusual manner and these groups arrive in resonator H during each pulse Nin proper phase to receive maximum acceleration from the field withinthe resonator. The accelerated electrons strike target 32 and produceX-rays as in Fig. 1.

Summarizing the above, current pulses pass through resonator 43 intoresonator ll' only at the close of each powerpulse delivered by circuit33, and are thus injected into resonator I! only when the oscillationsin resonator II have reached a maximum. When this pulsating current doesflow from resonator 43, the electrons are velocitygrouped in each pulsedue to the high frequency electric field between grids 45 and 46 7 andtheir subsequent passage through drift space 44, and the velocitygrouped electrons are given maximum acceleration when passing throughresonator l l. The proper phase of these grouped electrons with respectto the field in resonator 43' is assured by connection 59.

Electrons in large numbers therefore enter resonator I i when theoscillation voltage is such as to impart maximum acceleration to themand only a few electrons flow into resonator I I when the oscillationvoltage would give them less acceleration. The remaining conditions foraccelerating the electrons entering resonator H to high voltage are asdescribed for Fig. 1.

The provision of a plurality of loosely coupled secondaries puts all thesources of bias voltages at ground potential to avoid capacity effectsin the apparatus. The same result may be obtained in other ways, as forexample by supplying the pulses directly to the respective parts throughleads from a single secondary and using radio frequency choke coils inthe supply leads to keep the high frequency components from reachingback to the power sources.

Fig. 6 illustrates a further embodiment of the invention wherein alaterally swinging, lateralvelocity modulated electron beam is employedto introduce electrons into the X-ray generator resonator in such phasewith the high frequency oscillations therein that the electrons passthrough the resonator in largest numbers when the oscillation voltage ishighest.

Cathode is arranged to discharge electrons in pulses. similarly to themanner described in Fig. 5, through an entrance grid 62 into theinterior of a hollow metal resonator 63 having onposed reentrant lateralpoles 64 and 65 and an exit grid 66. Similar to Fig. 5, the smallerresonator 63 is coupled by concentric line and loop arrangements 59, iiito the interior of the large resonator 61, and this arrangement providesthat the electric field between poles B4 and 65, which is transverse tothe direction of flow of the electrons between grids 62 and 66, impartsalternate lateral velocities to the'electrons and thus causes the beamto swing laterally in accord with the oscillating frequency of thefield.

Resonator 63 is of the cylindrical shape obtainable by revolving thecross-section of the resonator 63 about an axis through the center A-Aof the reentrant poles 64 and B5. The electron beam passes therethroughat right angles to this axis or transversely to the electric fieldinstead of parallel thereto as in Fig. 5.

The swinging electron beam cyclically laterally traverses the top wallof resonator 61 as indicated and an entrance grid 50 is so located thatelectrons are injected into the resonator only at one extreme of lateraltravel of the beam. An inclined tube 53' having a focusing grid 66'opposite undue absorption of the X-rays.

grid 80 aids in attaining this operation. This extreme is of coursechosen so that the electrons enter resonator 61 only when theoscillation voltage in resonator 61 is in such phase as to acceleratethe electrons. Timing of the electron pulses with the recurrence of theoscillating field is the same as in Fig. 5.

The operation of resonator 61 in accelerating electrons to high voltageis the same as previously described for resonator II. The principles oflateral beam swinging employed in this embodiment of the invention inresonator 63 are preferably the same as described in U. S. LettersPatent No. 2,272,165, issued February3, 1942, to Varian et al., to whichreference is made for further detail.

Fig. 7 illustrates a further resonator construction 58 which may beemployed instead of resonators H, I l or 63 in the invention but whichis shown by way of example in the assembly of Fig. l. Resonator 68 is asubstantially bell-shaped hollow container having a top wall formed witha central depressed portion 69 surrounded by an annular entrance gridsection Ill. Smoother and focusing grid II is provided between entrancegrid 12 and central cathode 25, and annular cathode I6 is disposedwithin the evacuated enclosure I3' above the grid H.

The X-ray target 13 preferably comprises a thin coating of gold or someother heavy metal on the fiat internal bottom wall 14 of resonator 68.This coating provides for maximum production of X-rays when struck bythe. accelerated electrons, and is of suflicient thinness to avoid TheX-rays freely penetrate and emerge from the copper bottom wall of theresonator as indicated. An annular cooling passage member 15 is securedupon the bottom wall of resonator 68. Coating 13 may be used instead oftarget 32 in Figs. 1, 5 and 6.

\ Resonator 68 is a special development of the type of space resonatordisclosed in United States Letters Patent No. 2,269,456, issued January13, 1942, to William W. Hansen and myself, which employs only a singleenclosed resonant circuit dimensioned to maintain an oscillating fieldwithin the resonator and to effect electron grouping in an electron beampassing through the field parallel to electrostatic vector of the field.The distance between entrance grid Ill and bottom wall 14 is such thatthe electrons from cathode l8 entering at different points along thecycle of high frequency oscillations of the field become grouped duringtransit and deliver energy to the field at the resonant frequency of thefield before reaching wall I4. The distance from grid 10 to wall 14 inan operative embodiment was found to be such that the flight time of theelectrons between said grid 10 and said wall 14 is about two andone-quarter cycles, at the operating frequency.

Since the theory of operation of this resonator is fully developed andexplained in detail in said U. S. Patent No. 2,269,456, furtherdiscussion thereof here is unnecessary and reference is made to thispatent for further explanation. In the present invention it isessentially important only that resonator 68, instead of resonator H ofFig. 1, may be employed to accelerate electrons X-ray producingvelocities.

The considerations of design discussed in th early part of thisspecification apply as well t resonator 68, so that it is desirable toemploy a resonator having low shunt impedance to the driving voltage andlarge grid area for handling high entrance currents. This isaccomplished by resonator 68 which is essentially a figure of revolutionmade by revolving thecross-section shown about an axis a--a parallel toits electric vector so as to provide large annular grid 10.

As in Fig. 1, cathodes l8 and inject electrons into the resonator insynchronized pulses, having definite phase and duration relations andthe field cyclically maintained by the pulses from cathode l6 serves toaccelerate the electrons injected by cathode 25 to high velocity forstriking target 13. Except for the actual functioning of resonator 88 inproducing electron grouping, the operation and uses of the apparatus ofFig. '7 are the same as for Fig. 1.

The shape of resonator 68 is new and 01K s'uilicient advantage towarrant further attention. As above explained, it is a generallybell-shaped fig-- ure of revolution about axis a-a having a continuousannular entrance grid surrounding a central depression in the top wall.The bottom wall is fiat, completing a hollow metal structure shapedaccording to the above design characteristics.

In the invention, any of the cathode arrangements may be used as desiredwith any of the resonator structures shown, as the resultantcombinations may perform the intended function of the invention.

As many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope t .erecf, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

1. In apparatus for producing high velocity electrons, means providing ahigh frequency oscillating electromagnetic field recurring periodicallyat spaced intervals, and means passing an electron stream through saidfield only in such phrase with the recurrence of said field that theelectrons in .said stream are accelerated to substantially maximumvelocity.

2. In apparatus for accelerating electrons to high velocity, meansproducing an intermittent electromagnetic field oscillating at highfrequency, and means producing pulses of electrons and passing themthrough said field substantially only when the electrons are acceleratedduring passage through said field.

3. In ultra high frequency apparatus, means providing a high frequencyoscillating field recurrin periodically at spaced intervals, and meansinjecting pulses of electrons into said field substantially only whenthe oscillations of said field are at maximum level.

4. In apparatus for accelerating electrons to high velocity, hollowresonator means, means for periodically generating high frequencyoscillations within said resonator means, and means for periodicallyinjecting pulses of electrons into said resonator substantially onlywhen said oscillations are at maximum level.

5. In high frequency apparatus, a, resonator, means injecting pulses ofelectrons into said resonator to establish a recurrent high frequencyoscillating field therein, and means injecting further pulses ofelectrons into said resonator substantially only during the latterportions of said first pulses so that said field is given suflicienttime to build up to a, prescribed level before introduction of saidfurther pulses.

6. In a method of accelerating electrons to high velocity, the steps ofproviding an electromagnetic field oscillating at high frequency andrecurring at spaced intervals, and injecting pulses of electrons intosaid field substantially only in such phase therewith as to receiveacceleration therefrom.

7. In a method of accelerating electrons to high velocity, the steps ofproviding an electromagnetic field oscillating' at high frequency andrecurring at spaced time intervals, and injecting pulses of electronsinto said field only when the oscillations thereof have reachedsubstantially maximum level.

8. In apparatus for accelerating electrons to high velocity, meansproviding an electromagnetic field oscillating at high frequencyandrecurring at spaced intervals, means passing pulses of electrons throughsaid field in predetermined phase with the recurrence of said field, andmeans for introducing said electrons into said field in such relation asto receive maximum acceleration from said field.

9. In a 'method of accelerating electrons to high velocity, the steps ofproviding an electromagnetic field oscillating at high frequency andrecurring at spaced time intervals, providing pulses of electrons forinjection into said field in predetermined phase with the recurrence ofsaid field, and introducing the electrons of said pulses into said fieldin predetermined phase for acceleration by said field.

10. In a method of accelerating electrons to high velocity, the steps ofproviding an electromagnetic field oscillating at high frequency andrecurring at spaced intervals, providing pulses of electrons forinjection into said field only when said field is at'substantiallymaximum level,

and introducing said electrons into said field in proper phase to begiven maximum acceleration by said field.

11. In apparatus for accelerating electrons to high velocity, meansproviding an electromagnetic field oscillating at high frequency andrecurring at spaced intervals, means passing pulses of electrons throughsaid field in predetermined phase with the recurrence of said field, andmeans in said last-named means efi'ecting velocity grouping of theelectrons in each pulse prior to entering said field.

12. In apparatus for producing highly penetrating X-rays, a source ofintermittent power pulses, means energized by said source forperiodically building up a high frequency oscillating electromagneticfield, means energized by said source for periodically injecting pulsesof electrons into said field in such association that said electrons areaccelerated during passage through said field, and target means in thepath of said accelerated electrons adapted to produce penetrating X-raywhen impacted by said accelerated electrons.

13. In apparatus for producing high velocity electrons, a source ofintermittent power pulses, means energized by said source forperiodically building up a high frequency oscillating electromagneticfield, and means also energized by said source for periodicallyinjecting pulses of electrons into said field in such association thatsaid electrons are accelerated during passage through said field.

14. In apparatus for producing high speed electrons, an electricalconverter for transforming direct current pulses into high frequencyoscillating currents establishing a recurrent electromagnetic field,means for supplying energizing current pulses to said converter, meansenergized from said supply means producing an output proportional to therate of change of the current in said pulses, a source of electrons tobe injected into and accelerated by said field, and means coupling saidsource to be energized by said output.

'15. In apparatus for accelerating electrons to high velocity, electronaccelerating means pro-- viding an oscillatory electromagnetic field,means producing a stream of velocity modulated electrons forintroduction into said field, and means altering the potential of saidlast-named means as a unit with respect to said electron acceleratingmeans. I

16. In apparatus for accelerating electrons to high velocity comprisinga hollow resonator, a substantially annular hollow metal body withinsaid resonator, means producing and passing a substantially annular beamof electrons through said resonator and said body, and means producingand passing a second stream of electrons through the space surrounded bysaid hollow annular body, said space containing the electromagneticfield set up within said resonator.

17. In apparatus for accelerating electrons to high velocity, a hollowresonator, means producing and passing a substantially annular beam ofelectrons through said resonator for establishing an oscillatingelectromagnetic field therein, and means for producing and passing asecond beam of electrons through said resonator field in suchassociation that said second electron beam is subjected to largevelocity changes during passage through said field.

18. In apparatus for producing high velocity electrons, a hollowresonator of electrically conductive material having a substantiallycontinuous electron permeable wall section surrounding an intermediatewall portion having a further electron permeable section spaced fromsaid first section, and means producing and passing separate anddistinct synchronized electron streams through said sections.

19. In the apparatus defined in claim 18, said intermediate wall portionbeing depressed.

20. The method of producing high velocity electrons which comprisespassing an electron beam through the oscillating electromagnetic fieldof a hollow resonator, while subjecting said field to short excitationpulses of a duration equal to substantially is approximately equal tothe ratio of the duration 01' each power pulse to the interval betweenpulses, where V1 is the moderate excitation voltage and V0 is themaximum oscillation voltage during each pulse.

22. A resonator for high frequency apparatus comprising a hollowgenerally bell-shaped electrically conductive container having a wallclosing the smaller end of said container formed with a substantiallycontinuous electron permeable section surrounding an intermediate area,a spaced and distinct electron permeable section in said intermediatearea, and a wall across the larger end of said container facing saidelectron permeable sections.

23. In high frequency apparatus, a hollow resonator, pulsing means forexciting a high frequency oscillating field within said resonator to amoderate oscillation level, further pulsing means for exciting saidfield to peak value, and means correlating said individual pulsing meansso that said means for exciting the field to peak value becomeseffective only when said moderate oscillation level has been attained.

24. In a method of exciting a hollow resonator, the steps ofperiodically exciting said resonator to a moderate oscillation level,and then periodically exciting said resonator to peak value only whensaid moderate oscillation level has been attained.

25. In high frequency apparatus, a hollow resonator, means periodicallyproducing an oscillating field of moderate value within said resonator,and means periodically exciting said field to peak value.

26. In high frequency apparatus, a hollow resonator, means providingpower in pulses for periodically establishing an oscillating field ofmoderate value within said resonator, and means impressing furtherpulses of power on said resonator to raise said field to maximumoscillation, said first pulses being of appreciably longer duration thansaid further pulses.

27. Apparatus for highly exciting an oscillator for short periodscomprising means for applying high power pulses to said oscillator forrecurrent periods of short duration as compared to the interval betweenpulses, and means for applying lower power pulses to said oscillatorimmediately preceding said high power pulses.

28. In the apparatus defined in claim 27, said lower power pulses beingof appreciably longer duration than said high power pulses.

29. In apparatus for accelerating electrons to high velocity, meansproviding a recurrent electromagnetic field oscillating at highfrequency, means passing pulses of electrons through said field inpredetermined phase with the recurrence of said field, and means forintroducing said electrons into said field in such relation as toreceive maximum acceleration from said field, said last-named meanscomprising a hollow resonator oscillating at the same frequency as saidfield for velocity grouping the electrons in each pulse prior tointroduction into said field.

30. In apparatus for accelerating electrons to high velocity, meansproviding a recurrent electromagnetic field oscillating at highfrequency, means passing pulses of electrons through said field inpredetermined phase with the recurrence of said field, and means forintroducing said electrons into said field in such relation as toreceive maximum acceleration from said field, said lastnamed meanscomprising a hollow conductive resonator enclosing an oscillation spaceand feedback means coupling said space with the recurrent field.

31. In apparatus for accelerating electrons to high velocity, meansproviding a recurrent electromagnetic field oscillatingat highfrequency, means passing pulses of electrons in a stream through saidfield in predetermined phase with the recurrence of said field, andmeans for introducing saidelectrons into said field in such relation asto receive maximum acceleration from said field, said last-named meanscomprising means eflecting lateral swinging of the electron lowresonator providing a recurrent electromagnetic field oscillating athigh frequency, means passing pulses of electrons through said field inpredetermined phase with the recurrence of said field, and meansvforintroducing said electro into said field in such relation as to receivein I mum acceleration from said field, said last-named means comprisinga second hollow resonator in coaxial alignment with said first resonatorand oscillating at the same frequency as said field for velocitygrouping the electrons in each pulse prior to introduction into saidfield.

RUSSELL H. VARIAN.

